1. Field of the Invention
The present invention relates to a method for producing low cost Integrated Module Assembly (IMA) packages, and, more specifically, to a method for producing hermetic microwave integrated circuit (MIC) packages, wherein a waveguide window is formed on a surface of a non-hermetic substrate through the deposition and selective treatment of a metallic layer to produce a non-conductive hermetic protective layer between a waveguide and the Integrated Module Assembly circuitry.
2. Description of the Prior Art
Methods for producing Integrated Module Assembly (IMA) packages are generally known in the art. Typically, IMA package circuitry includes active devices which can include discrete transistors, capacitors, and monolithically microwave integrated circuits. The active devices on IMA substrates require protection from ambient moisture and other corrosives. Hermetic seals are typically required to achieve the desired protection over the large temperature ranges to which the hosts of many IMA packages are subjected. In addition, these hermetic seals may require non-electrically conductive properties if the seals are used for microwave or similar applications.
Presently, Integrated Module Assembly packages use substrates made of ceramic or similar inorganic materials to form hermetic seals over IMA substrate to waveguide interconnections. For example, when a microwave signal is transferred from the circuitry on an IMA substrate to a waveguide interconnection, ceramic substrates are used to cover the transition to the waveguide to protect the IMA circuitry from moisture. Further, because the ceramic substrate is non-electrically conductive, signals passing through the waveguide are not deflected.
Various methods are known for the use of metallic layers or films as a means for providing hermetic seals in IMA packages. An example of such a package is disclosed in U.S. Pat. No. 4,650,108. As illustrated in the '108 patent, a hermetic seal is formed by adhering a metal film to a surface of a substrate.
Additionally, in a context unrelated to the proposed invention, U.S. Pat. No. 4,922,323 discloses the treatment of metallic layers to form non-electrically conductive metallic layers which provide supports around electrical feedthrus.
The methods disclosed in the '323 and '108 patents do not attempt, however, to disclose methods for hermetic seals formed of electrically non-conductive metallic material to provide a moisture free environment for circuitry located on a substrate.
Finally, while inorganic substrates like ceramic have been successful at providing hermetic protection of delicate circuitry from moisture and other corrosives, ceramic and like substrates are expensive, more difficult to fabricate and tend to be heavier than other substrate materials. For these reasons, a substrate is needed which maintains the hermeticity of substrates like ceramic, is lighter, less expensive and can be formed to provide non-electrically conductive characteristics for use as hermetic seals in microwave integrated circuit applications.